High-Resolution Radio Maps of Wolf-Rayet Galaxies: Optically Thick H II Regions?

Details High-Resolution Radio Maps of Wolf-Rayet Galaxies: Optically Thick H II Regions? High-Resolution Radio Maps of Wolf-Rayet Galaxies: Optically Thick H II Regions?

Jul 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aj....120..244b&link_type=abstract

The Astronomical Journal, Volume 120, Issue 1, pp. 244-259.

Astronomy and Astrophysics

Astronomy

47

Galaxies: Starburst, Radio Emission Lines

Scientific paper

We present 20, 6, 3.6, and 2 cm Very Large Array maps and continuum-free Hα images of nine Wolf-Rayet galaxies. The radio maps were made with high spatial resolution to reject extended emission and to emphasize the star formation regions. The ratio of Hα to radio is at least a factor of 10 lower than predicted, indicating that the radio emission is from highly obscured sources. We map the radio spectral index, α (Sν~να), across each galaxy. In all the sample galaxies except one, the radio spectrum is much flatter than in spiral starburst galaxies, suggesting that free-free emission is more dominant in the dwarfs and that these are very young starbursts that have produced few supernovae. In many of the galaxies, there are regions where the spectrum rises from 6 to 2 cm. This requires that the radio emission be optically thick at wavelengths as short as 2 cm. In these optically thick regions, the emission measure, electron density, and ionizing fluxes must be very high, and they are probably the youngest parts of the starburst. The deduced ionization of these sources implies stellar content of hundreds to many thousands of O stars, which means that they may be responsible for a significant fraction of the total infrared luminosities of the galaxies. We discuss what these sources imply for the history and evolution of the starburst in each galaxy. Wise Observatory preprint 99/2.

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